High speed serial communication systems are specified at a high level in terms of maximum data and error rates. Such specifications are generally dictated by limitations in components forming the communication channel, including a transmitting device, a receiving device, and the transmission medium between the transmitting and receiving devices. The communication channel components distort fast edges corresponding to transitions in serial data signals, for example, by introducing frequency dependent magnitude loss and frequency dependent group delay due to nonlinear phase.
Generally, a serial data signal includes high and low voltage levels, e.g., corresponding to “1s” and “0s”, of the data modulated onto the serial data signal. A transition edge between a low voltage level and a high voltage level is referred to as a “rising edge,” and a transition edge between a high voltage level and a low voltage level is referred to as a “falling edge,” both of which may be considered as “fast edges” at high data rates. The time it takes for the serial data signal to fully transition from the low voltage level to the high voltage level may be referred to as “rise time,” and the time it takes for the serial data signal to fully transition from the high voltage level to the low voltage level may be referred to as “fall time.” For purposes of illustration, the description herein is directed to rising edge transitions and corresponding rise time, although the description may equally apply to falling edge transitions and corresponding fall time.
Exemplary distortions of rising edges of serial data signal are shown in FIGS. 1A and 1B, indicating the distorting effects of a communication channel with magnitude loss and with increasing group delay, respectively, although one communication channel may include both distortions. More particularly, FIG. 1A illustrates the effect on a rising edge of magnitude loss, where amplitudes of the frequencies composing the rising edge experience progressively more attenuation as frequency increases (while the group delay remains constant, for purposes of illustration). Ideally, a rising edge (particularly a fast edge) transitions from the low voltage level to the high voltage level substantially instantaneously (that is, rise time is approximately zero). The practical effect of magnitude loss on the rising edge in the time domain is an increase in rise time, indicated by shifting the substantially vertical rising edge 101 to a slower (sloped) transitioning rising edge 101′ occurring over an expanded period of time.
FIG. 1B illustrates the effect on a rising edge of frequency dependent increasing group delay in a communication channel, where the frequencies of the rising edge experience progressively more time delay as frequency increases (while magnitude remains constant, for purposes of illustration). Again, the practical effect on the rising edge in the time domain is an increase in rise time, indicated by shifting the substantially vertical rising edge 111 to a slower (sloped) transitioning rising edge 111′ occurring over an expanded period of time. Also, in the time domain, the higher frequency components present in the rising edge 111 are delayed more than the lower frequency components, thereby initially pushing the higher frequency components past the high voltage level, causing “overshoot” and subsequent oscillation (“ringing”) before settling at the high voltage level of the rising edge 111′.
In general, both magnitude loss and increasing group delay (or nonlinear phase) present in a communication channel impose an upper limit on the frequencies that can be transmitted without excessive distortion (e.g., excessive “eye” closure in an eye diagram) and acceptable error rate. Thus, there is a need to compensate for magnitude losses and increased group delay otherwise introduced by a communication channel.